Mac may have the most visible role of FW, but that doesn't mean his role has any authority. All of the behind the scenes crew, director, and producers are the ones who really make the show what it is. For the whole 7.62 issue, I can totally see the prop guy buying the wrong box of rounds, and on the day of the shoot the director just tells Mac that those are the rounds we have and those are the rounds we're shooting this scene with, end of story. Mac can't exactly say no to the director. Same with other errors. The editors don't know jack about guns. They piece together a 30 minute show from hours of footage. Some things are not going to line up right. They don't care as they know less than .0001% of the viewers are even going to notice.
Mac doesn't have any say in the script, editiing or directing. And the writers, director, and editors don't know anything about guns, but they still make the final call as to what goes in the show. As far as things like shooting so targets don't go down with "normal" weapons, but taking time and care to make good shots with the featured weapons, that is probably an intentional action on the part of the director. It's the directors job to create and produce an entertaining show that highlights the featured weapons as better. Mac can either play ball and follow the director and producers, or they can just fire him and find a new puppet. It's not really up to Mac.